Pre-tensioning of concrete is a method for improving the load carrying capacity of concrete structures. It can be used to produce beams, floors or bridges with longer spans and less deflection using thinner sections than is practical with ordinary reinforced concrete. Tendons used for pre-tensioning are generally made of high tensile steel cables and are used to provide an initial compressive load which produces a compressive stress that balances some or all of the tensile stress that the concrete member would otherwise experience due to a bending load. Pre-tensioning can be accomplished in three ways: pre-stressed concrete, and bonded or un-bonded post-tensioned concrete.
Pre-stressed concrete is cast around already tensioned cables. This method produces a good bond between the cable and concrete, which both protects the cable from corrosion and allows for direct transfer of loads between the cable and the concrete. The cured concrete adheres and bonds to the cables and when the load on the cables is released, much of it is transferred to the concrete as compression by static friction. However, it requires stout formwork and anchoring points between which the cable is stretched and held prior to the placement of the concrete. The cables are usually in a straight line unless deviators are installed in which case the cable will typically have straight segments. Most pre-tensioned concrete elements are prefabricated in a factory and must be transported to the construction site, which limits their size. Examples of pre-stressed elements include balcony elements, lintels, floor panels, double tees, beams and foundation piles.
Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring and curing the concrete. The concrete is typically cast around a duct, which may be of plastic, steel or aluminium. The ducts are often curved or draped to follow the profile where they will provide the greatest structural benefit. One or more cables are generally fished through the duct after the concrete is poured. Once the concrete has hardened, the cables are tensioned by hydraulic jacks that react against the concrete member itself. When the cables have been tensioned sufficiently, they are wedged or clamped in position to maintain tension in the cables and compression in the concrete after the jacks are removed. The duct is then filled with a grout to protect the cables from corrosion.
Un-bonded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete. To achieve this, each individual cable is typically coated with grease and enclosed by a plastic sheathing. In some cases the cable is a loose fit inside the sheath with the intention that the grease fills the space between the cable and the sheath. This construction can be formed by inserting the cable from one end into a pre-formed tubular sheath with the space between them being sufficient to allow the insertion to occur. In other cases the sheath is formed by covering the cable and surrounding grease with a strip of the covering material which is bent around the cable and then sealed or welded along a longitudinal seam to surround the cable.
In both cases the intention is that the cable is coated with grease and is sealed to prevent moisture entry and to ensure that the steel cable is maintained in a moisture free environment. In practical operation voids are typically present in this type of construction.
In another manufacturing system which is more typically used today for un-bonded mono-strand tendons, the cable is covered by grease and a plastic sheath is extruded onto the exterior of the cable as a tight envelope. In this case there are typically no voids and no path around the cable for moisture or other material to migrate along the sheath.
It is also intended that the sheath be continuous, complete and moisture impermeable. In practice the sheath is often damaged by its introduction into the forms, by failure to seal the ends of the sheath, or by the tensioning of the cable so that moisture can enter. Thus in the theoretical world, no moisture can enter into or migrate along the sheath so that no corrosion can occur. In practice corrosion typically does occur even to the extent of causing catastrophic failures if not detected and remediated.
The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded at the end of each cable. The main disadvantage over bonded post-tensioning is the fact that a cable can de-stress itself and burst out of the slab if damaged (such as during repair on the slab). The advantages of this system over bonded post-tensioning are:
a) The ability to individually adjust cables based on poor field conditions (For example: shifting a group of four cables around an opening by placing two to either side),
b) The procedure of post-stress grouting is eliminated, and
c) it is possible to de-stress the cables before attempting repair work.
In U.S. Pat. No. 5,365,779 (Vander Velde) issued Nov. 24, 1994 there is disclosed a method and apparatus for the corrosion condition evaluation of un-bonded pre-stressing elements in post-tension concrete structures. The method involves locating a pre-stressing element in the structure and providing at least two openings in the structure at positions along the length of the element. One of the openings is an inlet port and the other is an outlet port, each of the ports permitting communication with the gaseous environment within a conduit surrounding the pre-stressing element. The gaseous environment is accessed through the outlet port by extracting a sample of gas therethrough. The sample is then measured to determine its humidity and thereby evaluate the corrosion condition of the pre-stressing element between the inlet port and the outlet port. A method and apparatus is also provided for the on-site corrosion protection of the un-bonded pre-stressing elements whereby the gaseous environments within the conduits are cyclically pressurized with a dry gas for extraction of moisture which may lead to corrosion.
However as set forth in the above patent, the use of this method is limited to un-bonded post tensioning systems where there is enough space between the cable and its surrounding sheath to form a conduit to allow the passage of gas for sampling and for later drying if necessary or selected. WO 87/06958 assigned to Precision Dependability and Quality Testing discloses a method of treating a reinforced structure of masonry or cementitious material, such as concrete, to inhibit corrosion of the reinforcement. The method comprises inserting within the said material a vapour phase corrosion inhibitor so that the inhibitor migrates through the porous structure of the said material, and more-particularly along the interface between the said material and the reinforcement, to protect the reinforcement.
Again in this method there must be a channel or path around the reinforcement within the material to allow the passage of the vapour phase inhibitor.
However in regard to the other pre-tensioning methods defined above there is no channel or path for the passage of the protective fluid so that the above methods are not possible with such constructions.
The disclosures of the above patents are incorporated herein by reference.
This method of the present invention relates to the pre-tensioning system where the cable is intimately surrounded by the concrete itself.
This method of the present invention relates to the un-bonded post-tensioning system where the cable is contained within an extruded plastic sleeve extruded onto the cable and extending along the concrete member and there is provided an un-bonded filler material, generally grease, between the cable and the sleeve arranged to allow sliding of the cable within the sleeve during tensioning;
This method of the present invention relates to the bonded post-tensioning system where the cable is contained within a tubular container extending along the concrete member and the cable is intimately surrounded by a filler material, generally a cementitious grout, inserted into the tubular container after tensioning of the cable within the tubular container so that the grout is bonded intimately to the wires of the cable. Often multiple cables are installed in a single tubular container and the grout filler material fills and bonds intimately to all of the cables within the duct.
These methods now constitute a significant proportion of the installed pre-tensioned reinforcing systems so that a majority of the pre-tensioned structures cannot be protected using the methods of the above patents.